I think I found a bug. When I put 4-Aug-1978 for birthdate, and 41 for age, it said age and birthday don't match. It validated successfully on 42. Although maybe that's part of the game?
Not sure, I didn't want to do math so I just put in todays date and an age of 0. Worked!
At least one popular mobile game has a policy that the account's birthdate will not be changed under basically any circumstance, and child accounts are severely restricted compared to adult accounts. Parents wanting to take over their kid's account (happens more often than you'd think) end up seriously frustrated with the restrictions.